1. Field of the Invention
The present invention relates to a ball valve, in particular, it relates to a ball valve made of plastic and suitable for utilization in the food industry for transporting perishable liquids or purified water.
2. Description of the Prior Art
The conventional ball valve, for example, as disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 58-89656, comprises, as illustrated in FIG. 1, a valve body 1 of a cylindrical shape, a ball 2 accommodated in the valve body 1 and having a through-channel in the central portion, a pair of unions 3, each attached to opposite ends of the valve body 1 so as to be in slidable contact with the ball through a seat 9, and a pair of body caps 4 each connected to the outer end of the unions 3 by means of a cap nut 5. According to the above arrangement, a liquid passage to be connected to the up- and downstream piping (not shown) is formed through the valve by a central bore provided in each of these elements, as shown in FIG. 1. The ball 2 can be rotated by a handle 8 and a stem 7 through 90.degree. about its axis to shut off the liquid passage, as illustrated in FIG. 2.
According to the above construction, an annular cavity A is left between the union 3 and the ball 2. In use, liquid transported through the valve gradually leaks into the cavity A through a contact area between a seat 9 and the surface of the ball 2 and is entrapped therein. This entrapped liquid may freeze and expand in winter, resulting in breakage of the valve. This causes another serious problem in that, when the valve is incorporated in the piping system for food industry, the entrapped liquid may putrefy because of the presence of bacteria and thus contaminate the entire system. Further, when the transported liquid is purified water, the purity of the entrapped water tends to be degraded over the lapse of a period of time.
Another such cavity having a tendency to entrap the liquid resides in the valve structure other than the above cavity A. That is, there is a second cavity B between the end surfaces of the union 3 and the body cap 4. The cavity B is inherently formed because an O-ring 10 for sealing a connection between the union 3 and the body cap 4 must have a thickness larger than the depth of a groove 11 accommodating the O-ring 10 to ensure tight sealing. The second cavity B directly entraps the liquid from the liquid passage, which liquid entrapment results in the same drawbacks as those of the first cavity A, even if the effects of the former are less than those of the latter.
In some cases, for example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-37160, a further cavity C may exist in the inside wall of the outer end of the union formed by dents engageable with a special tool for screwing the union relative to the valve body. The cavity C also has drawbacks similar to those of the cavities A and B.